This invention relates generally to mechanical joints and, more particularly, to a collar for controlling stress in a mechanical joint subjected to movement and high temperatures.
A steam reformer, also known as a steam-methane reformer, is used to produce hydrogen gas by passing a mixture of methane and steam through a heated catalyst. The catalyst is normally contained within tubes positioned within a heated chamber. In the design and construction of steam reformers and other similar equipment, the gas which is produced is transferred from the catalyst tubes to an outlet manifold by a plurality of small tubes, commonly referred to as "pigtails". The pigtails are subjected to significant bending stresses due in part to thermal expansion and contraction which occurs during start-up and shut-down of the reformer, and in part due to the bowing or bending which frequently occurs in the relatively-long catalyst tubes.
The pigtails are connected at their ends to the catalyst tube and the outlet manifold, respectfully, by extension elements, commonly called weldolets, which are conical-shaped elements having a central bore aligned with holes provided in the walls of the catalyst tube and the outlet manifold. The weldolets are welded to the surfaces of the catalyst tube and the manifold, and the ends of the pigtails, in turn, are welded to the weldolet.
Numerous failures have been experienced in the pigtails at the points where they are welded to the weldolets. These failures are the result of the concentration of stress occurring at the tip of the weld which joins the pigtail to the weldolet. These failures, which are normally cracks in the wall of the pigtails, are propagated by a combination of thermal fatigue and grain boundary oxidation caused by the stress concentration aggravated by the bending stresses.
The weld which joins the pigtail to the weldolet, in addition to creating a mechanical notch with the concomitant stress concentration associated therewith, also degrades the yield strength of the material in the heat-affected zone of the weld, thus also causing a metallurgical notch.